The invention relates generally to cooling systems, and in particular to a system and method for cooling a superconductive device.
Superconductivity is a phenomenon observed in several metals and ceramic materials. When these materials are cooled to temperatures ranging from near absolute zero (−459 degrees Fahrenheit, 0 degrees Kelvin, −273 degrees Celsius) to liquid nitrogen temperatures (−321 F, 77 K, −196 C), or even higher, they have no electrical resistance. Because these materials have no electrical resistance, they can carry large amounts of electrical current for long periods of time without losing energy as heat. This property has implications for electrical power transmission and for electrical devices, such as motors and generators. The temperature at which electrical resistance is zero is called the critical temperature or transition temperature and is different for different materials. Typically, critical temperatures are achieved by cooling superconductive materials with a cryogen, such as liquid helium or liquid nitrogen.
Devices such as motors and generators employ superconductors to improve their operating efficiency. Motors and generators typically include a stator mounted in a housing, and a rotor, which is disposed within the stator and can rotate during operation. In a generator, the rotor is coupled to a prime mover that rotates the rotor, producing a rotating magnetic field that induces a current in the stator. The current produced in the stator may be used to supply power to an electrical grid or other distribution network. In a motor, the stator produces a rotating magnetic field that interacts with the magnetic field produced by the rotor coils to induce rotation of the rotor. In practice, a motor may be reconfigured to function as a generator, or vice versa.
Conventionally, copper conductors are used to form the rotor coils. However, the electrical resistance of the copper conductors is sufficiently large to produce substantial resistive heat losses in the rotor coil of the generator or motor. These heat losses reduce the efficiency of the device. In response to the losses caused by conventional copper conductors, superconductors have been developed for use as rotor coils.
In devices employing a superconductive rotor coil, the rotor coil is typically cooled to reduce the temperature of the coil below its transition temperature. Typically, a cryogenic fluid or cryogen, such as liquid helium or liquid nitrogen, as discussed above, is provided to cool the rotor coils. The cryogenic fluid absorbs heat from the superconductive rotor coil, and maintains the rotor coil below the transition temperature and in a superconducting state. The cryogenic fluid is typically supplied by a refrigeration system that operates to maintain the fluid in a liquid state.
However, a power outage, a failure of the refrigeration system, or a maintenance shutdown of the refrigeration system may cause an interruption in the supply of the cryogenic fluid to the device. This interruption can result in ultimately raising the temperature of the coil beyond the transition temperature, and loss of superconductivity.
Accordingly, there is a need for a technique that enables uninterrupted supply of cryogenic fluid to superconducting devices, such as motors and generators.